This project was initiated to develop a neuropharmacological approach to the mechanisms regulating circadian rhythms and the effects of light. Rhythms of indoleamine metabolism in the rat and chick pineal, activity rhythms in rat and hamster, and aspects of biochemistry in the rat and bovine retina have been examined. The effects of light on the circadian rhythms in rat pineal can be mimicked by injections of carbachol which appears to act on a nicotinic receptor. Carbachol also affects the activity rhythms of rats and hamsters. Diazepam receptors are present in the rat retina. Light and darkness modulate phospholipid metabolism in the retina.